Book - The brain of the tiger salamander 9
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Herrick CJ. The Brain of the Tiger Salamander (1948) The University Of Chicago Press, Chicago, Illinois.
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Part I. General Description and Interpretation 1. Salamander Brains | 2. Form and Brain Subdivisions | 3. Histological Structure | 4. Regional Analysis | 5. Functional Analysis, Central and Peripheral | 6. Physiological Interpretations | VII. The Origin and Significance of Cerebral Cortex | VIII. General Principles of Morphogenesis Part 2. Survey of Internal Structure 9. Spinal Cord and Bulbo-spinal Junction | 10. Cranial Nerves | 11. Medulla Oblongata | 12. Cerebellum | 13. Isthmus | 14. Interpeduncular Nucleus | 15. Midbrain | 16. Optic and Visual-motor Systems | 17. Diencephalon | 18. Habenula and Connections | 19. Cerebral Hemispheres | 20. Systems of Fibers | 21. Commissures | Bibliography | Illustrations | salamander
Part Ii Survey Of Internal Structure
Introductory Note
The first part of this work includes a schematic outhne of the organization of the amphibian brain and discussion of some morphological and physiological principles suggested by this inquiry. In Part II, evidence is presented in sufficient detail to document the conclusions and principles summarized in Part I. This material is arranged by topographic regions, as these are listed in chapter iv. These descriptions are supplementary to those in Part I, and each topic should be read in connection with the corresponding passages of the preceding text.
Chapter IX Spinal Cord and Bulbospinal Junction
The Spinal Cord and its Nerves
Though the spinal cord is not included in this survey, some features of its upper segments must be considered here because of their connections with the brain and particularly with the complicated structure at the bulbo-spinal junction. The spinal cords of urodeles have not been adequately described, and our material is not suitable for this purpose. Early stages of development of Amblystoma have been described by Coghill, and the cord of larval Salamandra by van Gehuchten ('97). A wide variety of experimental studies of development involving the spinal cord have been reported by others.
THE SENSORY ZONE
The slender dorsal gray column is continuous with the nuclei at the bulbar junction, to be described shortly. These cells are activated by the dorsal spinal root fibers and the spinal V and spinal vestibular roots. A few of them in and near the mid-plane are a spinal continuation of the commissural nucleus of Cajal and are probably visceral sensory in function. This supposition is supported by Sosa's description ('45) of similar cells in the septum dorsale of mammals and birds, which he regards as spinal representatives of Cajal's nucleus.
Most of the dorsal root fibers immediately upon entrance bifurcate
into descending and ascending branches, and the latter in the upper
segments comprise most of the massive dorsal funiculi, with which
some other fibers are mingled, notably those of the descending
vestibular root and bulbar correlation tracts a and h of Kingsbury.
The large spinal V root lies ventrally of these funiculi, and below this
is a dorsolateral funiculus, containing fibers of the spinal lemniscus,
spino-cerebellar tract, and other fibers of spinal and bulbar correlation. Many of these dorsolateral fibers decussate, descending from
the dorsal gray as internal arcuates.
THE INTERMEDIATE ZONE
The intermediate zone is not clearly defined, its gray substance being continuous with that of the motor zone. In the alba the neuropil of the reticular formation is less well developed than in the medulla oblongata.
THE MOTOR ZONE
The motor zone of the cord is continuous with that of the medulla oblongata, with no recognizable boundary; and throughout these regions the peripheral motor neurons are mingled with co-ordinating neurons, and they often resemble the latter so closely that they cannot be distinguished unless their axons are seen to enter the motor roots.
THE SPINAL NERVES
The upper spinal nerves are modified. The first usually has no sensory root or ganglion. The arrangement of the motor roots of the first and second pairs is exceedingly variable. The nervus accessorius and the nervus hypoglossus are not separately differentiated. The primordia of the former are represented in the lowermost vagal rootlets, which emerge from the lateral aspect of the medulla oblongata; and the primordia of the latter are in the first and second spinal nerves, the ventral roots of which emerge at the ventral surface. In one specimen the lowest vagal root was seen to emerge at the level of the calamus scriptorius, but usually it is at a more rostral level. The lowest root of the first spinal nerve usually emerges in the region of the calamus, and the first root of this nerve at Variable distances rostrally.
The first spinal nerve of Salamandra as described by Francis
('34, p. 159) agrees with that of Amblystoma. On page 161 he quotes
Goodrich, who has shown that the urodele hypoglossus innervates
muscles derived from the ventral outgrowths of the second, third,
and fourth myotomes and that "the hypoglossus of Amphibia and
Amniota may certainly be considered as homologous, although not
necessarily composed of the same segmental nerves."
The neurons of the ventral horns of gray include tegmental elements, and motor cells which give rise to peripheral fibers ('44&, fig. 10; van Gehuchten, '97). Both types may have very large, muchbranched dendrites, which in the larva ramify through almost the entire cross-sectional area of the cord and may cross to the other side in the ventral commissure. In the adult animal, internuncial connections within the dorsal and ventral zones and between these provide for co-ordinated spinal reflexes; but all movements of the trunk and limbs are subject to further control from bulbar and other higher centers. The details of the structural apparatus by means of which these ordered movements are effected remain obscure. At the inception of motihty in the embryo the first neuromotor responses to stimulation are mass movements, and the apparatus of local reflexes matures later (Coghill, '29). This implies that integrative functions of total-pattern type mature earlier than do the partial patterns of the local reflexes. Coghill's studies revealed a transitory system of peripheral and central connections in the early stages of the development of motility when mass movements prevail, followed by radical changes as the action system becomes more complicated.
Before the spinal ganglia are functionally mature, a series of transitory giant ganglion cells (Rohon-Beard cells) within the cord send
peripheral processes out to skin and myotomes and central processes,
which effect connection with the neuromotor elements. The transitory cells subsequently disappear and are replaced by the more specialized elements of the spinal ganglia.
Intramedullary cells of sensory type were observed by Humphrey
('44) in the spinal cords of human embryos. Two types of bipolar
sensory cells appear in embryos of 5 mm., one of which is transient
and is regarded as homologous with the Rohon-Beard cells. The other
type persists to functional stages, and at the beginning of motility
{'i2.5 mm.) many of these are changing to a unipolar shape and resemble cells of the spinal ganglia. These intramedullary unipolar
cells are found in embryos of from 16 to 144 mm. in length, and are
regarded as functioning components of the dorsal roots in the early
stages of motility. Youngstrom ('44) also reports the occurrence of
sensory cells within the spinal cords of human embryos of from 19 to
63 mm. These cells are in the mantle layer and resemble those regarded by Humphrey as comparable with Rohon-Beard cells. Similar
intramedullary cells of sensory type have been seen by many others
in embryos and adults to accompany root fibers of spinal and cranial
nerves (see Pearson, '45, for illustrations) ; and it is probable that
these are all derived from the neural crest, like the mesencephalic
nucleus of the V nerve (as Piatt, '45, has demonstrated).
On the motor side of the arc two types of peripheral neurons were
described by Coghill ('26, Paper VI) and Youngstrom ('40) : (1) The
thick primary fibers appear first in ontogeny and course for long
distances in the ventral funiculus before emergence. The first ventral
root fibers arise as collaterals of these longitudinal axons. (2) Thinner
secondary fibers, which appear later, pass out from the gray of the
cord more nearlv transverselv- The Rohon-Beard cells are centrally so connected with the primary motor cells as to evoke mass movement of the musculature of the trunk in response to adequate stimulation of any kind. In subsequent stages central connections between
spinal ganglion cells and secondary motor neurons are made, and
these are regarded as provision for execution of local reflexes. The
primary motor neurons persist in adult Amblystoma. They occur in
larval anurans but disappear at metamorphosis (Youngstrom, '38).
Humphrey ('44) describes cells in the spinal cords of very young
human embryos, which she believes are surviving vestiges of primary
motor neurons of amphibian type.
lu our sections of the adult the ventral spinal roots contain fibers of
primary and secondary type. The primary root fibers are thick and
heavily myelinated centrally and peripherally. Some of the thinner
secondary fibers are well myelinated, and many of them seem to lose
their myelin as they emerge from the spinal cord. Coghill ('26, Paper
VI, p. 135) reported that in early swimming stages "a single fiber
may innervate an entire myotome, and branches of these same fibers
form the earliest nerves to limbs and tongue." At this early stage,
however, the musculature of the limb bud is still an undifferentiated
primordium. Youngstrom confirmed these observations and expressed the opinion that the limb musculature, like that of the trunk,
has a double innervation of both primary and secondary fibers; but
no details of the distribution of these fibers in the definitive limb
musculature are given. More recently, Yntema ('43a, p. 331) says of
the primary fibers in larvae of from 12 to 19 mm. in snout-anal
length that "typically, they supply the myotomic musculature. In
addition, fibers of this kind run to muscles of the extremities"; but
again details of their distribution in the limb are lacking. In a personal communication he adds: 'T have found evidence for the distribution of primary motor fibers to at least some muscles of the
girdles of larvae, and have seen larger fibers which appear to be primary in the limbs themselves."
In the frog, with an action system very different from that of Amblj^stoma, the development of these nerves shows corresponding differences, for, as mentioned above, Youngstrom ('38) found that larval frogs have primary and secondary fibers like those of Amblystoma ; but in the adult frog the primary fibers have completely disappeared. The opinion expressed by Taylor ('44) that in frog larvae the primary fibers do not enter the limbs may have no bearing on the innervation of limbs in Amblvstoma because of the radical difference in the neuromuscular apparatus of these species. The primary fibers evidently are concerned with massive movements of the trunk musculature. The significance of the two sorts of fibers in the innervation of the limbs is still obscure. In Amblystoma the number of primary motor fibers is not markedly reduced by removal of the early undifi^erentiated neural crest, while secondary fibers are, as a rule, greatly reduced in number, the growth of the latter being dependent on the presence of sheath cells and the former not (Yntema, '43a). It will be of interest to learn whether the primary motor fibers have functions in the embryogenesis of Amblystoma comparable with those postulated for the "pioneer motor neurons" observed in the bird by Hamburger and Keefe ('44, p. 237).
THE BULBO-SPINAL JUNCTION
Little need be added here to the general description of this important region in chapter iv and to the details of structure and connections recently pubUshed ('446). The topography as seen in transverse Weigert section is shown in figure 87. If the calamus scriptorius is taken as the arbitrary boundary between spinal cord and brain, this junctional region in Amblystoma may be considered to comprise the segments of the first and second pairs of spinal nerves, the second below the calamus and the first above. The entire length of the first spinal segment overlaps the lower vagus region of the medulla oblongata.
In the sensory zone the somatic sensory systems of the neurons of
the dorsal gray columns are somewhat enlarged to form the nucleus
of the dorsal funiculi, which extends far forward in the lower vagus
region. Medially of this is the much larger collection of compactly
arranged smaller cells of visceral-gustatory function^ — the commissural nucleus of Cajal. This nucleus extends downward from the
calamus for a distance of about one spinal segment, below which
visceral sensory function is represented by scattered cells in the
dorsal median raphe. Above the calamus the commissural nucleus
merges insensibly with the nucleus of the fasciculus solitarius.
The funicular nucleus is regarded as comparable with the external
cuneate nucleus of mammals rather than with the nuclei of the f.
gracilis and f. cuneatus, since Amblystoma has no medial lemniscus
('446, p. 318). The arrangement and connections of the commissural
nucleus are similar to those of man.
Secondary fibers from the funicular nucleus (many of them myelinated) pass downward as internal arcuate fibers to the spinal cord
and medulla oblongata, some uncrossed and some decussating in the
ventral commissure. Other crossed fibers join the tractus spinocerebellaris and the spinal lemniscus (fig. 3). The secondary fibers
from the commissural nucleus are unmyelinated. Some of them are
internal arcuates, which distribute to neighboring parts of the spinal
cord and medulla oblongata of the same and of the opposite side;
and some pass directly laterally to the pial surface, where they turn
rostrad in tr. visceralis ascendens (fig. 8; '44&, figs. 10, 11, 12, tr.v.a.)
to reach the superior visceral nucleus in the isthmus and the ventrolateral neuropil of the peduncle. Some further details about the connections of these nuclei are in the next two chapters.
The region of the calamus scriptorius is evidently an important
center of correlation and integration of general somatic and visceralgustatory sensibility of the entire body, with efferent discharge directly to the motor zone and also to higher centers of sensory correlation. Here root fibers of cutaneous and deep sensibility from the
head, trunk, and limbs; of vestibular and lateral-line sensibility; and
of gustatory and visceral sensibility converge into a common pool,
which is the first integrating center of these functional systems to
mature in ontogeny.
